Bisazo dyes and their use in liquid crystalline materials

ABSTRACT

Novel compounds of the general formula I ##STR1## where R is hydrogen, unsubstituted or substituted alkyl, aralkyl, cycloalkyl, aryl, alkoxy, aralkoxy, cycloalkyloxy, aryloxy, alkylthio, aralkylthio or cycloalkylthio, Y is a radical of the formula ##STR2## and the rings A and B may be further substituted or fused to a benzene ring, and R 1  and R 2  are each hydrogen or unsubstituted or substituted alkyl, aralkyl or cycloalkyl, or together with the nitrogen atom form piperidino, pyrrolidino or morpholino, and R 3  is hydrogen, alkyl, or cycloalkyl, are particularly useful as dyes in liquid-crystalline media.

SUMMARY OF THE INVENTION

The present invention relates to compounds of the general formula I##STR3## where R is hydrogen, unsubstituted or substituted alkyl,aralkyl, cycloalkyl, aryl, alkoxy, aralkoxy, cycloalkyloxy, aryloxy,alkylthio, aralkylthio or cycloalkylthio, Y is a radical of the formula##STR4## and the rings A and B may be further substituted or fused to abenzene ring, and R¹ and R² are each hydrogen or unsubstituted orsubstituted alkyl, aralkyl or cycloalkyl or together with the nitrogenatom form piperidino, pyrrolidino or morpholino, and R³ is hydrogen,alkyl or cycloalkyl.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the extinctions E.sup.∥ and E.sup.⊥ for the compound ofExample 18 in the liquid crystalline mixture ZLI 1957/5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Examples of suitable substituents for the ring A are chlorine, bromine,methyl, ethyl, methoxy, ethoxy, acetylamino, propionylamino or a fusedbenzene ring.

The ring B may carry as substituents, for example, chlorine, bromine,methyl, ethyl, methoxy, ethoxy, acetylamino, propionylamino or a fusedbenzene ring.

In addition to being hydrogen, R¹ and R² are each, for example, methyl,ethyl, propyl, butyl, hexyl, dodecyl, cyclohexyl, 2-phenylethyl,2-cyanoethyl, 2-hydroxyethyl, 2-acetoxyethyl, 2-butyryloxyethyl, allyl,phenylmethyl or 4-butylphenylmethyl.

In addition to being hydrogen, R³ is, for example, methyl, ethyl, butylor cyclohexyl.

R is hydrogen or, for example, phenyl, phenylmethyl, phenylethyl,phenoxy, phenoxymethyl, phenoxyethyl, phenylmethylthio orphenylethylthio, the stated radicals being unsubstituted or substitutedby C₁ -C₂₄ -alkyl, ε₁ -C₂₄ -alkoxy, C₁ -C₂₄ -alkoxycarbonyl, C₁ -C₂₄-acyloxy, C₅ -C₇ -cycloalkyl, 4-(C₁ -C₁₂ -alkylcyclohexyl), phenyl,chlorine or bromine, or R is C₅ -C₇ -cycloalkyl or is C₁ -C₂₄ -alkylthioor C₁ -C₂₄ -alkyl which is unsubstituted or substituted by hydroxyl, C₁-C₂₄ -alkoxy, C₁ -C₂₄ -alkoxycarbonyl, C₁ -C₂₄ -alkyl- ordialkylcarbamyl or C₁ -C₂₄ -acyloxy.

Examples of radicals R in addition to those stated above are 4-(C₁ -C₁₂-alkyl)-phenyl, 4-cyclohexylphenyl, 4-(4'-C₁ -C₇-alkylcyclohexyl)-phenyl, 4-(C₁ -C₁₂ -alkyl)phenylmethyl, 4-(C₁ -C₁₂-alkyl)-phenylethyl, 4-(C₁ -C₁₂ -alkyl)-phenylmethylthio, 4-(C₁ -C₁₂-alkoxy)-phenylmethylthio, 4-(C₁ -C₁₂ -alkoxycarbonyl)-phenylmethylthio,4-(C₁ -C₁₂ -alkanoyloxy)-phenylmethylthio, 4-cyclohexylphenylmethylthio,4-(4'-C₁ -C₇ -alkylcyclohexyl)-phenylmethylthio and 4-(C₁ -C₁₂-alkyl)-phenylethylthio.

Examples of preferred substituents R are methyl, ethyl, propyl, butyl,2-butyl, 3-heptyl, octyl, nonyl, dodecyl, methylthio, ethylthio,propylthio, butylthio, isobutylthio, tert.-butylthio, pentylthio,2-methylbutylthio, hexylthio, 2-ethylhexylthio, octylthio, nonylthio,isononylthio, dodecylthio, phenylmethylthio, 4-(C₁ -C₁₂-alkyl)-phenylmethylthio, 4-cyclohexylphenylmethylthio, 4-(C₁ -C₇-alkylcyclohexyl)-phenylmethylthio, 4-(C₁ -C₁₂ -alkyl)-phenyl and4-(4'-C₁ -C₇ -alkylcyclohexyl)-phenyl.

The compounds of the formula I are particularly useful as pleochroicdyes in electrooptical liquid crystal displays of the guest-host type,and for the preparation of colored polarization films.

Liquid crystalline materials which contain pleochroic dyes are used indisplays. The principles of the novel use are known, and are describedin, for example, H. Kelker and R. Hatz, Handbook of Liquid Crystals,page 611 et seq (1980), R. J. Cox, Mol. Cryst. Liq. Cryst. 55 (1979), 51et seq., and L. Pauls and G. Schwarz, Elektronik 14 (1982), 66 et seq.Further literature which describes the novel use in detail is referredto in the stated publications.

Dyes for liquid crystalline mixtures must meet a number of requirements(cf. for example J. Constant et al., J. Phys. D: Appl. Phys. 11 (1978),479 et seq., F. Jones and T. J. Reeve, Mol. Cryst. Liq. Cryst. 60(1980), 99 et seq., and European Patent Nos. 43,904, 55,838 and 65,869).They must not ionize in an electric field, must have a very high molarextinction coefficient ε and be readily soluble in the liquid crystalmatrix used, must be chemically and in particular photochemicallystable, and must possess a degree of order S as far as possible greaterthan 0.75 in the particular nematic phase in order to achieve goodcontrast of the guest-host display.

Violet and blue dyes which meet these requirements are predominantlythose of the anthraquinone class (cf. for example Nos. EP-A-56492,EP-A-91225, DE-A-3028593, EP-A-54217 and DE-A-2902177).

Azo dyes known to date, in particular violet and blue ones, generallyhave the disadvantage that the degree of order, the solubility and thelight stability are inadequate (cf. for example G. W. Gray, Chimia. 34(1980), 47 et seq.).

Surprisingly, the novel violet and blue dyes have a very good degree oforder coupled with high solubilities and in many cases adequate lightstabilities in the particular liquid crystal matrix (eg. ZLI 1840, ZLI1957/5, ZLI 2452, ZLI 2585 from Merck).

The use of dichroic dyes for the preparation of polarization films isdescribed by, for example, R. Mizoguchi et al. in Displays, 4 (1983),201 et seq., where further literature is also referred to.

The compounds of the formula I can be prepared by diazotizing a compoundof the formula II ##STR5## with, for example, nitrosylsulfuric acid inglacical acetic acid, and then reacting the product with a couplingcomponent of the formula ##STR6## to give a compound of the formula##STR7##

By diazotizing the monoazo compounds and then coupling the product witha compound of the formula HY, the compounds of the formula I areobtained.

The compounds of the formula II are prepared by a conventional method(cf. for example J. Goerdeler, Chem. Ber. 87 (1954), 57, J. Goerdeler,A. Huppertz and K. Wember Chem. Ber. 87 (1954), 69, J. Goerdeler and F.Bechlars, Chem. Ber. 88 (1955), 843, J. Goerdeler and M. Willig, Chem.Ber. 88 (1955), 1071, J. Goerdeler, J. Ohm and O. Tegtmeyer, Chem. Ber.89 (1956), 1535 and J. Goerdeler and P. Linden, Chem. Ber. 89 (1956),2742).

The dyes can be purified by chromatography over silica gel using, forexample, a toluene/ethyl acetate mixture as the mobile phase. They arethen recrystallized, for example from toluene. The purity is monitoredby thin-layer chromatography, HPLC and elemental analysis.

A typical method for preparing the dyes of the formula I is described inExample 1 below. Parts and percentages are by weight, unless statedotherwise.

Of particular importance are compounds of the formula Ia ##STR8## whereB¹ is hydrogen, unsubstituted or substituted alkyl, aralkyl, cycloalkyl,aryl, alkylthio, aralkylthio or cycloalkylthio, B² and B³ independentlyof one another are each hydrogen, methoxy or methyl, and B⁴ and B⁵independently of one another are each hydrogen, C₁ -C₂₄ -alkyl,2-phenylethyl, unsubstituted or substituted benzyl or cyclohexyl, ortogether with the nitrogen atom form pyrrolidino, piperidino ormorpholino.

Other particularly important compounds are those of the formula Ib##STR9## where B¹, B² and B³ have the meanings given for formula Ia andB⁶ is hydrogen, C₁ -C₂₄ -alkyl, 2-phenylethyl or unsubstituted orsubstituted cyclohexyl or benzyl.

B⁶ is particularly preferably hydrogen, C₁ -C₂₄ -alkyl, cyclohexyl,2-phenylethyl, benzyl, 4-(C₁ -C₂₄ -alkyl)benzyl, 4-cyclohexylbenzyl or4-(4'-C₁ -C₁₂ -alkylcyclohexyl)-benzyl.

Other preferred compounds are those of the formula Ic ##STR10## where B¹to B⁵ have the meanings given for formula Ia.

GENERAL METHOD OF PREPARATION EXAMPLE 1 ##STR11##

10.8 parts of 36% strength nitrosylsulfuric acid are added, at 5° C., toa mixture of 7.4 parts of 3-octyl-5-amino-1,2,4-thiadiazole (prepared asdescribed by J. Goerdeler in Chem. Ber. 89 (1956), 2742), 70 parts ofglacial acetic acid and 30 parts of propionic acid, and the mixture isstirred for a further 3 hours at 5° C.

The diazonium salt solution is then added, while cooling with ice, to asolution of 6.3 parts of sodium anilinomethanesulfonate, 1 part ofamidosulfonic acid and 100 parts of water. The mixture is allowed toreach room temperature, after which it is stirred for a further 3 hours,and the resulting precipitate is filtered off under suction. The moistresidue is taken up in 100 parts of ethanol and 30 parts of concentratedhydrochloric acid, and the mixture is heated at the boil for 0.5 hour.When the mixture has cooled, the monoazo dye of the formula ##STR12##obtained in the form of the hydrochloride, is filtered off under suctionand washed with water, then with a concentrated aqueous sodium acetatesolution and then again with water. The filtration residue which hasbeen thoroughly sucked dry is taken up in 70 parts of glacial aceticacid and 30 parts of propionic acid, after which first 9 parts of a 23%strength aqueous sodium nitrite solution and then 10 parts ofconcentrated hydrochloric acid are added to this solution at 5° C., andthe mixture is stirred for 4 hours at this temperature. Thereafter, thediazonium salt solution is added to a mixture of 4.8 parts ofN-butyl-N-methylaniline, 50 parts of isobutanol, 100 parts of asaturated aqueous sodium acetate solution and 200 parts of ice water.After the mixture has warmed up to room temperature, the resulting dyeis filtered off under suction, washed with methanol and then with waterand dried. 8 parts (51% of theory) of the crude dye of Example 1 areobtained, this dye being purified by chromatography over silica gel(Merck, silica gel 60, 0.063-0.0200 mm) using one part of a 40:1toluene/ethyl acetate mixture as the eluent, followed byrecrystallization from toluene/cyclohexane.

Mp.: 122° C., λ max (CH₂ Cl₂): 565 nm.

    __________________________________________________________________________    Ex-                                                                           am-                                                       λ max        ple                                                    M.p.                                                                             (CH.sub.2                                                                     Cl.sub.2)           __________________________________________________________________________     2                                                                                ##STR13##                                          232°                                                                      575 nm               3                                                                                ##STR14##                                          226°                                                                      578 nm               4                                                                                ##STR15##                                          182°                                                                      577 nm               5                                                                                ##STR16##                                          224°                                                                      580 nm               6                                                                                ##STR17##                                          134°                                                                      578 nm               7                                                                                ##STR18##                                          210°                                                                      598 nm               8                                                                                ##STR19##                                          146°                                                                      578 nm               9                                                                                ##STR20##                                          218°                                                                      600 nm              10                                                                                ##STR21##                                          240°                                                                      623 nm              11                                                                                ##STR22##                                           181°                                                                     549 nm              12                                                                                ##STR23##                                          227°                                                                      597 nm              13                                                                                ##STR24##                                          185°                                                                      600 nm              14                                                                                ##STR25##                                          155°                                                                      600 nm              15                                                                                ##STR26##                                          165°                                                                      592 nm              16                                                                                ##STR27##                                          237°                                                                      602 nm              17                                                                                ##STR28##                                          235°                                                                      605 nm              18                                                                                ##STR29##                                          176°                                                                      588 nm              19                                                                                ##STR30##                                          206°                                                                      607 nm              20                                                                                ##STR31##                                          179°                                                                      604 nm              21                                                                                ##STR32##                                          132°                                                                      593 nm              22                                                                                ##STR33##                                           85°                                                                      558 nm              23                                                                                ##STR34##                                          115°                                                                      588 nm              24                                                                                ##STR35##                                          121°                                                                      600                 __________________________________________________________________________                                                              nm              

    __________________________________________________________________________     ##STR36##                                                                                                 M.p.                                                                             λ max                                  Example                                                                             R.sup.1   R.sup.2      °C.                                                                       (CH.sub.2 Cl.sub.2)                           __________________________________________________________________________    25    CH.sub.3  C.sub.8 H.sub.17                                                                           181                                                                              603 nm                                        26    CH.sub.3  C.sub.12 H.sub.25                                                                          142                                                                              606 nm                                        27    C.sub.2 H.sub.5                                                                         C.sub.8 H.sub.17                                                                           140                                                                              605 nm                                        28    C.sub.2 H.sub.5                                                                         C.sub.12 H.sub.25                                                                          142                                                                              604 nm                                        29    C.sub.3 H.sub.7                                                                         C.sub.8 H.sub.17                                                                           143                                                                              603 nm                                        30    C.sub.3 H.sub.7                                                                         C.sub.12 H.sub.25                                                                          139                                                                              603 nm                                        31    C.sub.4 H.sub.9                                                                         C.sub.8 H.sub.17                                                                           158                                                                              603 nm                                        32    C.sub.4 H.sub.9                                                                         C.sub.12 H.sub.25                                                                          145                                                                              603 nm                                        33    C.sub.5 H.sub.11                                                                        C.sub.8 H.sub.17                                                                           138                                                                              603 nm                                        34    C.sub. 5 H.sub.11                                                                       C.sub.12 H.sub.25                                                                          132                                                                              603 nm                                        35    C.sub.7 H.sub.15                                                                        C.sub.12 H.sub.25                                                                          113                                                                              605 nm                                        36    C.sub.8 H.sub.17                                                                        C.sub.8 H.sub.17                                                                           120                                                                              602 nm                                        37    C.sub.8 H.sub.17                                                                        C.sub.12 H.sub.25                                                                          113                                                                              605 nm                                        38                                                                                   ##STR37##                                                                              C.sub.2 H.sub.5                                                                            160                                                                              600 nm                                        39    C.sub.12 H.sub.25                                                                       C.sub.8 H.sub.17                                                                           137                                                                              603 nm                                        40    C.sub.12 H.sub.25                                                                       C.sub.12 H.sub.25                                                                          112                                                                              603 nm                                        41    C.sub.8 H.sub.17                                                                         ##STR38##   131                                                                              598 nm                                        42    C.sub.8 H.sub.17                                                                         ##STR39##   159                                                                              594 nm                                        43    C.sub.8 H.sub.17                                                                         ##STR40##   137                                                                              592 nm                                        __________________________________________________________________________

    __________________________________________________________________________     ##STR41##                                                                    Example     R.sup.3   R.sup.4   M.p. °C.                                                                    λ max (CH.sub.2 Cl.sub.2)         __________________________________________________________________________    44          H         H         164  588 nm                                   45          CH.sub.3  H         180  593 nm                                   46          CH.sub.3  CH.sub.3  177  593 nm                                   __________________________________________________________________________

                  TABLE 1                                                         ______________________________________                                                  Degree of                                                                     order S    Solubility L                                                       (at room   (at room     Liquid crystal                              Example   temperature)                                                                             temperature) mixture                                     ______________________________________                                        1         0.72       1.5%         ZLI 1957/5                                  4         0.73       0.9%         ZLI 1840                                    5         0.74       0.3%         ZLI 1840                                    8         0.73       1.5%         ZLI 1957/5                                            0.76       --           ZLI 2452                                    12        0.72       0.7%         ZLI 1840                                    15        0.77       1.4%         ZLI 1957/5                                            0.81       2.8%         ZLI 2452                                              0.74       2.8%         ZLI 2585                                    18        0.79       0.9%         ZLI 1957/5                                  ______________________________________                                    

The degree of order S measured at room temperature, the solubility Llikewise determined at room temperature, and the Merck liquid crystalmixtures used are summarized in Table 1 for Examples 1, 4, 5, 8, 12, 15and 18.

The values for the degree of order and the solubility which have beenmeasured at room temperature in ZLI 2452 are summarized in Table 2 forsome of the Examples 19-46.

                  TABLE 2                                                         ______________________________________                                                   Degree of order S                                                                          Solubility L                                                     (at room tempera-                                                                          (at room tempera-                                     Example    ture)        ture)                                                 ______________________________________                                        19         0.75         0.6%                                                  22         0.77         1.7%                                                  26         0.76         1.3%                                                  35         0.79         0.6%                                                  36         0.77         0.8%                                                  38         0.74         >5%                                                   40         0.77         1.2%                                                  41         0.74         0.6%                                                  42         0.80         0.4%                                                  43         0.71         3.2%                                                  44         0.81         2.6%                                                  45         0.81         2.4%                                                  46         0.80         0.8%                                                  ______________________________________                                    

The degree of order S was determined on the basis of the known equation##EQU1## in a commercial measuring cell with a homogeneous orientation(polyimide). The dichroic ratio CR was determined by measuring theextinctions E.sup.∥ (measured with light polarized parallel to thepreferred direction of the nematic phase) and E.sup.⊥ (measured withlight polarized perpendicular to the preferred direction of the nematicphase) and using the relationship CR=E.sup.∥ /E.sup.⊥, the dyeconcentrations being chosen so that E.sup.∥ was from 1 to 2. Themeasurements were carried out using an Acta CIII spectrophotometer fromBeckmann.

FIG. 1 shows the extinctions E.sup.∥ and E.sup.⊥ for the compound ofExample 18 in ZLI 1957/5.

The solubility was determined as described below. 50 mg of theparticular dye was stirred for 1 week at room temperature in 1 ml ofliquid crystal, the saturated solution was centrifuged off from theresidue, and the solubility was determined by comparison of extinctions.

The light stability of the dyes in the particular liquid crystal wasdetermined by high-speed exposure of the solution in the measuring cellin a Suntest apparatus (Hanau) at 25° C. The dyes exhibited goodphotostability, particularly when a UV protective coating was used.

We claim:
 1. A liquid crystal composition, comprising a bisazo dye whichcontains a 1,2,4-thiadizole ring, said bisazo dye being of the formulaI: ##STR42## wherein: R is hydrogen, or phenyl, phenylmethyl,phenylethyl, phenoxy, phenoxymethyl, phenoxyethyl, phenylmethylthio, orphenylethylthio radical, wherein the said radicals are eitherunsubstituted or substituted by C₁ -C₂₄ -alkyl, C₁ -C₂₄ -alkoxy, C₁ -C₂₄-alkoxycarbonyl, C₁ -C₂₄ -acyloxy, C₅ -C₇ -cycloalkyl, 4-(C₁ -C₁₂-alkylcyclohexyl), phenyl, chlorine or bromine, orR is C₅ -C₇-cycloalkyl or a C₁ -C₂₄ -alkylthio or C₁ -C₂₄ -alkyl, each of which isunsubstituted or substituted by hydroxyl, C₁ -C₂₄ -alkoxy, C₁ -C₂₄-alkoxycarbonyl, C₁ -C₂₄ -alkylcarbamyl, C₁ -C₂₄ -dialkylcarbamyl, or C₁-C₂₄ -acyloxy, Y is a radical of the formula: ##STR43## ring A isunsubstituted or substituted by chlorine, bromine, methyl, ethyl,methoxy, ethoxy, acetylamino, propionylamino, or ring A is fused to abenzene ring, ring B is unsubstituted or substituted by chlorine,bromine, methyl, ethyl, methoxy, ethyoxy, acetylamino, propionylamino,or ring B is fused to a benzene ring; R₁ and R₂ are each independentlyhydrogen, unsubstituted C₁ -C₂₄ alkyl, C₁ -C₂₄ alkyl substituted bycyano, hydroxy, acetoxy, or butyryloxy substituent, cyclohexyl,2-phenylethyl, unsubstituted benzyl, benzyl substituted by C₁ -C₂₄-alkyl, cyclohexyl or C₁ -C₁₂ -alkyl-cyclohexyl substituent, allyl, ortogether with the nitrogen atom R₁ and R₂ form a piperidino, pyrrolidinoor morpholino radical; and, R₃ is hydrogen, C₁ -C₂₄ -alkyl, orcyclohexyl.
 2. The composition of claim 1, wherein R₁ and R₂ are eachindependently hydrogen, methyl, ethyl, propyl, butyl, hexyl, dodecyl,cyclohexyl, 2-phenylethyl, 2-cyanoethyl, 2-hydroxyethyl, 2-acetoxyethyl,2-butyryloxyethyl, allyl, phenylmethyl, or 4-butylphenylmethyl; and R₃is hydrogen, methyl, ethyl, butyl or cyclohexyl.
 3. The composition ofclaim 1, comprising a bisazo dye of the formula: ##STR44## wherein: B¹is hydrogen, unsubstituted or substituted alkyl, aralkyl, cycloalkyl,aryl, alkylthio, aralkylthio or cycloalkylthio;B² and B³ areindependently hydrogen, methoxy, or methyl; and B⁴ and B⁵ independentlyare hydrogen, C₁ -C₂₄ -alkyl, 2-phenylethyl, unsubstituted orsubstituted benzyl, or cyclohexyl, or together with the nitrogen atom,B⁴ and B⁵ form a pyrrolidino, a piperidino, or a morpholino radical. 4.The composition of claim 1, comprising a bisazo dye of the formula:##STR45## wherein: B¹ is hydrogen, unsubstituted or substituted alkyl,aralkyl, cycloalkyl, aryl, alkylthio, aralkylthio or cycloalkylthio;B²and B³ are each independently hydrogen, methoxy, or methyl; and, B⁶ ishydrogen, C₁ -C₂₄ -alkyl, 2-phenylethyl, unsubstituted cyclohexyl,substituted cyclohexyl, unsubstituted benzyl, or substituted benzyl. 5.The composition of claim 1, comprising a bisazo dye of the formula##STR46## wherein: B¹ is hydrogen, unsubstituted or substituted alkyl,aralkyl, cycloalkyl, aryl, alkylthio, aralkylthio, or cycloalkylthio;B²and B³ are each independently hydrogen, methoxy, or methyl; and, B⁶ ishydrogen, C₁ -C₂₄ -alkyl, cyclohexyl, 2-phenylethyl, benzyl, 4-(C₁ -C₂₄-alkyl)benzyl, 4-cyclohexylbenzyl, or 4-(4'-C₁ -C₁₂-alkylcyclohexyl)benzyl.
 6. The composition of claim 1, comprising abisazo dye of the formula ##STR47## wherein: B¹ is hydrogen,unsubstituted or substituted alkyl, aralkyl, cycloalkyl, aryl,alkylthio, aralkylthio, or cycloalkylthio;B² and B³ are eachindependently hydrogen, methoxy, or methyl; and, B⁴ and B⁵ are eachindependently hydrogen, C₁ -C₂₄ -alkyl, 2-phenylethyl, unsubstitutedbenzyl, substituted benzyl, unsubstituted cyclohexyl or substitutedcyclohexyl, or together with the nitrogen atom to which they are bound,B⁴ and B⁵ form a pyrrolidino, piperidino, or morpholino radical.
 7. Thecomposition of claim 1, whereinR is 4-(C₁ -C₂₄ -alkyl)phenyl,4-cyclohexylphenyl, 4-(4'-C₁ -C₇ -alkylcyclohexyl)phenyl, 4-(C₁ -C₁₂-alkyl)phenylmethyl, 4-(C₁ -C₂₁ -alkyl)phenylethyl, 4-(C₁ -C₁₂-alkyl)phenylmethylthio, 4-(C₁ -C₁₂ -alkoxy)phenylmethylthio, 4-(C₁ -C₁₂-alkoxycarbonyl)phenylmethylthio, 4-(C₁ -C₁₂-alkanoyloxy)phenylmethylthio, 4-cyclohexyl-phenylmethylthio, 4-(4'-C₁-C₇ -alkylcyclohexyl)phenylmethylthio or 4-(C₁ -C₁₂-alkyl)phenylethylthio.
 8. The composition of claim 1, wherein R₁ and R₂are independently C₇ -C₁₁ -aralkyl.